Download Key Relational and SQL Concepts

Overview of Key Relational and SQL Concepts (Part 2 of 3)
Why Relational Databases?
Relational model first proposed in 1970 by Dr E F (Ted) Codd in the paper ‘A
relational model of data for large shared data banks’.
Purpose

Achieve program/data independence

Treat data in a disciplined way
- Apply rigour of mathematics
- Use set theory

Improve programmer productivity
Implementation
At first considered impractical, but in the late 1970s prototype System R developed
by IBM and during 1980s commercial products began to appear, in particular Oracle
from Oracle Corporation.
What is a Relational Database?
A relational database is made up of relations (tables) in which data are stored.
A relation (table) is a 2-dimensional structure made up of attributes (columns) and
tuples (rows).
Relation
A relation is a table that obeys the following rules:

There are no duplicate rows in the table.

The order of the rows is immaterial.

The order of the columns is immaterial.

Each attribute value is atomic, ie each cell can contain one and only one data
value.
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Example of a Table (Relation)
ANIMAL
ANAME
Candice
Zona
Sam
Elmer
Leonard
AFAMILY
Camel
Zebra
Snake
Elephant
Lion
WEIGHT
1800
900
5
5000
1200
Relational Database Terminology
Relation
a table with rows and columns
Tuple
a row of a relation
Attribute
a named column of a relation
Primary key
a unique identifier for each row in a relation
Domain
the set of allowable values for a column
Degree
the number of columns in a relation
Cardinality
the number of rows in a relation
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ANIMAL
ANAME
Candice
Zona
Sam
Elmer
Leonard
AFAMILY
Camel
Zebra
Snake
Elephant
Lion
WEIGHT
1800
900
5
5000
1200
Primary and Foreign Keys
A primary key is a unique identifier for each row in a table.
It may consist of one or more columns.
ANIMAL
ANO
CA1
ZE4
SN1
EL3
LI2
ANAME
Candice
Zona
Sam
Elmer
Leonard
AFAMILY
Camel
Zebra
Snake
Elephant
Lion
WEIGHT
1800
900
5
5000
1200
No part of the primary key may have a null value. This is known as the entity
integrity rule.
Each table contains data about one entity.
ANIMAL-FOOD
ANO
CA1
CA1
ZE4
SN1
SN1
EL3
LI2
LI2
FOOD
Hay
Buns
Brush
Mice
People
Leaves
People
Meat
You may need to combine 2 or more tables to find out a particular piece of
information, eg you may want a report on the animal name, family and food.
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You relate the data in one table to the data in another through foreign keys.
A foreign key is a column or columns in one table which reference(s) a primary key
column or columns in another table.
Values in a foreign key must match an existing value in the primary key or be NULL.
This is known as the referential integrity rule.
ANIMAL
ANO
CA1
ZE4
SN1
EL3
LI2
ANIMAL-FOOD
ANAME
Candice
Zona
Sam
Elmer
Leonard
AFAMILY
Camel
Zebra
Snake
Elephant
Lion
WEIGHT
1800
900
5
5000
1200
ANO
CA1
CA1
ZE4
SN1
SN1
EL3
LI2
LI2
FOOD
Hay
Buns
Brush
Mice
People
Leaves
People
Meat
ANO in the ANIMAL-FOOD table is part of the primary key and also a foreign key.
Database Languages
SQL, often known as a query language, is a combined DDL, DML and DCL used
with relational databases.



A Data Definition Language (DDL) is used to specify the data in a database.
A Data Manipulation Language (DML) is used to access the data in a
database.
A Data Control Language (DCL) is used to control access to the data in a
database.
Characteristics of SQL

The DDL statements define database objects, eg databases, tables, views,
indexes, users, constraints, user-defined data types:
CREATE, DROP, ALTER

The DML statements manipulate data:
SELECT, INSERT, DELETE, UPDATE

The DCL statements control access to data:
GRANT, DENY, REVOKE
Transact-SQL is the SQL Server implementation of SQL.
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SQL Server Object Names
Standard Identifiers

Can contain from one to 128 characters, including letters, symbols (_ @ or #)
and numbers.

No embedded spaces are allowed.

The first character must be alphabetic.

A name beginning with @ denotes a local variable or parameter.

A name beginning with # denotes a temporary table or procedure.

A name beginning with ## denotes a global temporary object.
NB:
Names for temporary objects shouldn’t exceed 116 characters including # or
## as SQL Server gives them an internal numeric suffix.
SQL Server Object Names (contd)
Delimited Identifiers
Do not comply with the rules for standard identifiers and must, therefore, always be
delimited.
You can use delimited identifiers when:

Names contain embedded spaces.

Reserved words are used for object names or portions of object names.
You must enclose delimited identifiers in square brackets or quotation marks when
you use them in Transact-SQL statements, eg:
SELECT * FROM [Blanks In Table Name]
SELECT * FROM “Blanks In Table Name”
NB:
You can always use bracketed delimiters but can only use quotation marks if
the SET QUOTED_IDENTIFIER option is on.
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SQL – DML Statements
The SELECT Statement
A query, ie a request for information to be retrieved from the database.
The SELECT statement is made up of a number of clauses, with a minimum of two:
SELECT
FROM
the columns to retrieve
the table(s) which contain those columns

The clauses may be entered on the same line or on separate lines.

You can enter statements in UPPER CASE, lower case or a MiXtUrE of the
two. However, reserved words are, by convention, entered in UPPER CASE.
Selecting All Columns and All Rows
To display all the information from a table called Student:
SELECT * FROM Student;
Selecting Specific Columns and All Rows
To select some of the columns and all the rows:
SELECT Studentno, Surname FROM Student;
The order of column names in a SELECT statement determines the order in which
the columns are displayed.
SELECT Surname, Studentno FROM Student;
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Selecting Specific Rows
Requires the WHERE clause following the FROM clause:
SELECT
FROM
WHERE
the columns to retrieve
the table(s) which contain those columns
certain conditions are met
SELECT * FROM Course
WHERE Courseno = 30;
SELECT Courseno, Coursename
FROM Course
WHERE Coursename = 'Computing';
Comparison Operators
Operator
Meaning
=
!= or <>
>
>=
<
<=
BETWEEN … AND …
IN (list)
LIKE
IS NULL
equal to
not equal to
greater than
greater than or equal to
less than
less than or equal to
between two values
any of a list of values
match a character pattern
is a null value
To negate the last four operators, use the operator NOT, ie NOT BETWEEN, NOT
IN, NOT LIKE and IS NOT NULL.
Examples
SELECT * FROM Course
WHERE Coursetype <> 'BSc';
SELECT Studentno, Surname FROM Student
WHERE Dob < '01/26/77';
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SELECT * FROM Student
WHERE Surname > 'M';
SELECT Tutorno, Job FROM Tutor
WHERE Salary >= 20000 AND Salary <=25000;
SELECT Tutorno, Job FROM Tutor
WHERE Salary BETWEEN 20000 AND 25000;
SELECT Tutorno, Job FROM Tutor
WHERE Salary NOT BETWEEN 20000 AND 25000;
SELECT Courseno, Coursename FROM Course
WHERE Courseno IN (30, 40, 50, 90);
SELECT Courseno, Coursename FROM Course
WHERE Coursename
NOT IN (‘Electronics’, ‘Computing’);
SELECT * FROM Course
WHERE Coursename LIKE 'Comp%';
SELECT Courseno, Studentno
FROM CourseStudent
WHERE Studymode LIKE '_T';
NB:
Each _ represents one character.
characters.
The % represents any number of
SELECT Tutorno, Studentno
FROM TutorStudent
WHERE Studentno NOT LIKE '_ _D%';
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Using AND
SELECT Tutorno, Job FROM Tutor
WHERE Salary >= 20000 AND Salary <=25000;
SELECT * FROM CourseStudent
WHERE Courseno = 50 AND Studymode <> 'FT';
A SELECT statement with AND only retrieves a row or rows if both parts of the
WHERE clause are true.
Using OR
A SELECT statement with OR retrieves a row or rows if either part of the WHERE
clause is true.
SELECT * FROM CourseStudent
WHERE Courseno = 50 OR Studymode <> 'FT';
SELECT Courseno, Coursename FROM Course
WHERE Courseno = 30 OR Courseno = 40;
Operator Precedence
When AND and OR appear in the same WHERE clause, AND has a higher precedence
than OR, ie all the ANDs are performed first, then all the ORs.
SELECT * FROM Tutor
WHERE Job = ‘Lecturer' AND Salary > 26788
OR Job = 'Senior Lecturer' AND Salary < 30058
OR Job = 'Principal Lecturer' AND Salary < 30058;
All the comparison operators have equal precedence, then the order of precedence
is:
NOT
AND
OR
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Changing Precedence
To override the rules of precedence, place the part of the expression you want
evaluated first in parentheses ().
Example
SELECT * FROM Tutor
WHERE Salary > 26788 AND Salary < 30058
AND (Job = 'Lecturer'
OR Job = 'Senior Lecturer'
OR Job = 'Principal Lecturer');
If in doubt, add parentheses to clarify your SELECT statements and ensure the correct
results.
Eliminating Duplicate Rows
The default display of a result from a query is all rows, including duplicate rows.
To eliminate duplicate rows, use the DISTINCT keyword in the SELECT clause, eg:
SELECT DISTINCT Coursename
FROM Course;
SELECT DISTINCT Job
FROM Tutor;
SELECT DISTINCT Courseno, Coursename
FROM Course;
Controlling the Order of Displayed Rows
You can control the order in which rows are displayed by adding the ORDER BY
clause as the last clause of a SELECT statement.
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ORDER BY lets you order rows in ascending order, in descending order and by
multiple columns.
SELECT * FROM Course
ORDER BY Coursename;
SELECT Tutorno, Job, Salary FROM Tutor
ORDER BY Salary DESC;
SELECT * FROM Student
ORDER BY Surname, Fname;
SELECT Tutorno, Job, Salary FROM Tutor
WHERE Salary BETWEEN 20000 AND 25000
ORDER BY Job, Salary DESC;
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